The invention relates to the treatment of liquid waste sludge, and more particularly to the dewatering and pasteurization of liquid waste sludge.
Many municipalities produce liquid sludge as an end product of sewage treatment. This sludge may contain hazardous pathogens, but can be turned into a beneficial and benign solid product, suitable for use as residential or agricultural fertilizer, by dewatering and treating for pathogen deactivation. One of the most common methods for deactivating liquid sludge or dewatered sludge solids is heat treatment to pasteurize the sludge. In the absence of complete pasteurization of the sludge, it is also desirable to reduce vector transmission of pathogens, which can result from attraction of rodents, birds or insects to the sludge. While municipal waste sludge is focused on primarily herein, the same concerns may arise from other types of sludge, such as fish and other food processing waste sludge, that require pathogen reduction.
Many governmental regulatory agencies have established, or are in the process, of establishing standards for the degree of treatment required of sludge before it can be safely and legally recycled or otherwise disposed. In the United States, these regulations are set forth in 40 C.F.R. 503.32-503.33 and publication EPA/832/R-93/003 xe2x80x9cA Plain English Guide to the EPA Part 503 Biosolids Rulexe2x80x9d (collectively, hereinafter xe2x80x9cEPA Part 503xe2x80x9d), which sets out rules and guidelines for the treatment of liquid waste sludge established by the Environmental Protection Agency (EPA). Reference herein to these US Federal Regulatory standards are to be understood to refer to similar or corresponding technical or governmental standards in other jurisdictions.
During sludge treatment, dewatering may conventionally be carried out in a screw press. U.S. Pat. No. 3,939,763 issued to Sato and assigned to Fukoku Kogyo Company Limited, the disclosure of which is hereby expressly incorporated by reference, discloses one such continuous screw press. The press includes a rotatable screw shaft, a screw drum secured to the screw shaft, a screw blade spirally arranged on the outer surface of the screw drum, an outer drum having a number of small filtering holes, an inlet for introducing raw materials to be pressed into a space between the screw drum and outer drum and an outlet for discharging pressed materials. The continuous press further includes steam introduction circuitry for heating the screw drum and blade from the inside of the screw drum through the screw shaft. While heating the screw in this fashion may aid dewatering, the degree of heating provided by this device is substantially insufficient for pasteurization during the average residence time of the sludge within the continuously operating screw press.
Heat treatment of sludge for pasteurization typically is carried out after dewatering. U.S. Pat. No. 4,380,496 to Maffet makes a passing reference to heating sludge during dewatering, stating xe2x80x9cHowever, it has recently been discovered that heat may be advantageously applied during dewatering of certain secondary sludges.xe2x80x9d (In Col. 7, Lines 55-57.) Maffet also states, xe2x80x9cThe heat may be applied by a heating element in contact with the upper surface of the porous wall and should heat the sludge to an average temperature of not over 80xc2x0 C.xe2x80x9d (Col. 7, Lines 58-60).
However, there are several problems with the approach of Maffet. While Maffet applies heat during dewatering, Maffet does not provide for simultaneous pasteurization and dewatering of liquid waste sludge. One problem has to due with the retention time of the screw of Maffet. Table 5-3 of EPA Part 503 requires that, for biosolids with 7% solids or greater, the xe2x80x9cTemperature of biosolids must be 50xc2x0 C. or higher for 20 minutes or longerxe2x80x9d. If the temperature is lower than 72xc2x0 C., the retention time must be greater than 20 minutes as determined by a formula discussed in the detailed description. In screw presses, the retention time of the sludge in the press is inversely related to the screw speed (rpm). As the screw speed is increased, the retention time decreases. As the screw speed decreases, the retention time increases. Higher screw speeds result in shorter retention times. Lower screw speeds result in longer retention time. Assuming 100% conveying efficiency, the retention time can be calculated by dividing the number of flights on the screw by the screw speed (rpm). Maffet describes a xe2x80x9chighly effectivexe2x80x9d screw design in column 8, line 28, with a total of seven flights of the screw conveyor used within the porous wall. Maffet describes the screw speed of his apparatus in column 7, line 41, as xe2x80x9cranging from about 10 to about 150 rpm, or even more rapidly if desiredxe2x80x9d. Maximum retention time of the sludge in the porous wall section of the Maffet press will be at the minimum speed, (10 rpm) so 7 flights divided by 10 rpm results in a 0.7 minute retention time. Being very conservative and assuming 50% conveying efficiency (the sludge spirals with the screw rather than in a straight line through the press), and assuming the number of flights is doubled to 14, the retention time is calculated to be 2.8 minutes. Thus, the retention time in Maffet""s press is not sufficient to pasteurize the sludge.
Further, the heating mechanism disclosed in Maffet is insufficient to raise the temperature of the liquid waste sludge to a sufficient degree for pasteurization. Maffet discloses heating the sludge xe2x80x9cby a heating element in contact with the upper surface of the porous wall.xe2x80x9d (Col. 7, Lines 58-60). The externally applied heat must heat the screen, which then must heat the outer sludge layer, and then the heat must penetrate down through the sludge mass down to the screw. While the heat is trying to flow inwards, cold water from the sludge is flowing outwards from the screw shell, out through the sludge, out through the porous wall and past the heating elements. The water from the sludge will move faster outwards than the heat can move inwards. Heat from the heating elements will be carried away with the water being pressed from the sludge. Thus, there are serious thermodynamic problems with his method of using xe2x80x9ca heating element in contact with the upper surface of the porous wallxe2x80x9d to heat the entire sludge mass which would prevent this method from working at the screw speeds/retention times disclosed.
The present invention provides a process for dewatering and reducing pathogens in liquid waste sludge having a liquid component and a solids component. The liquid waste sludge is fed into a screw press. The liquid waste sludge is simultaneously heated and dewatered in the screw press for a sufficient period of time and at a predetermined temperature for the high solids, dewatered sludge that is discharged from the screw press to be completely or nearly pasteurized at the time of discharge.
In another embodiment of the present invention, a process is provided for dewatering and reducing pathogens in liquid waste sludge having a liquid component and a sludge solids component. Alkaline material is added to the liquid waste sludge. Flocculent may then be mixed with the liquid waste sludge. The liquid waste sludge is then fed into a screw press having a screw. The screw simultaneously heats and dewaters the liquid waste sludge for a sufficient period of time and at a predetermined temperature for the dewatered sludge discharged from the screw press to be completely or nearly pasteurized.
In a further aspect of the invention, flocculent, or a combination of flocculent and coagulant, may be added to the liquid waste sludge prior to simultaneous heating and dewatering the liquid waste sludge.
In a still further aspect of the invention, the pasteurized and dewatered sludge solids are discharged from the screw press having a water content between 90% and 40% by weight, i.e., a total solid content of 10-60%.
In a still further aspect of the invention, the liquid waste sludge may be prethickened prior to the simultaneous heating and dewatering step by removing at least a portion of the liquid from the liquid waste sludge.
The present invention is also directed to an apparatus for dewatering, pasteurizing, and vector attraction reduction in liquid waste sludge, having a liquid component and a sludge solids component. The apparatus includes an alkaline introduction station, a flocculent introduction station and a screw press. A first conduit supplies liquid waste sludge to the alkaline introduction station. A second conduit supplies alkaline material to the alkaline introduction station, where the alkaline material is combined with the liquid waste sludge. The screw press has a liquid waste sludge inlet, a sludge solids outlet, an outer screen, and a screw rotatably located within the outer screen. The screen allows the liquid component to pass through and retains a majority of the sludge solids component. The screw has a heating mechanism for heating the liquid waste sludge. A third conduit supplies alkaline sludge mixture to the screw press inlet. A flocculent introduction station supplies flocculent to the alkaline sludge mixture in the third conduit, prior to the liquid waste sludge being introduced into the screw press.
In a still further aspect of the invention, the screw press further includes a second heating mechanism which may be a heated drum with a perforated or non-perforated inner wall in place of a portion of the screw press outer screen.
In a still further aspect of the invention, the apparatus for dewatering, pasteurizing, and vector attraction reduction in liquid waste sludge further includes a prethickener device located in the third conduit for removing a portion of the liquid from the liquid waste sludge prior to entry into the screw press.
The present invention thus provides processes and apparatus that enable the simultaneous dewatering and heating of liquid waste sludge, where the heat applied to the sludge while the sludge passes through the screw press for dewatering is sufficient to pasteurize the sludge solids component. In the preferred embodiment, a device constructed in accordance with the present invention heats liquid waste sludge from the inside utilizing a heated screw. Thus the flow of water radially outward from the sludge and the heat conduction flow are in the same direction. The liquid waste sludge next to the screw is heated as the liquid waste sludge is compressed between the screw and a screw press screen. The hot water migrates outward from the screw through the liquid waste sludge and out the screen. This flow aids in heat flow from the screw to the liquid waste sludge near the porous wall. Further, the screw press of the present invention operates at a screw speed range, with a given spiral pitch, to achieve a sludge retention time range allowing the sludge to come up to the required temperature for pasteurization and then be slowly conveyed at that temperature for the required amount of time for pasteurization.